There has been proposed a method for capping a Cu electrical interconnection with a metal film in order to improve the reliability of the Cu electrical interconnection. For instance, there have been known a selective film-forming method while making use of the plating method and a method for forming a metal-capping film according to the selective W-CVD method (see, for instance, Japanese Un-Examined Patent Publication Hei 10-229054 (Claims)).
When forming Cu electrical interconnections, the selective W-CVD method is, for instance, carried out, as shown in the attached FIG. 1 as a process flow diagram, by filling, with a Cu-film serving as a lower Cu electrical interconnection, structures such as holes and trenches formed on a substrate provided thereon with an electrical insulating film, according to the plating method (FIG. 1(a)); scraping off the excess Cu-film through the CMP (FIG. 1(b)); cleaning off the contaminants present on the electrical insulating film and the Cu electrical interconnection through the wet washing (FIG. 1(c)); and then selectively forming a capping film on the lower Cu electrical interconnection (FIG. 1(d-2)). In this respect, it is common that after the completion of this selective film-formation, an electrical insulating film is further formed thereon for the production of an upper Cu-electrical interconnection (FIG. 1(e)); the electrical insulating film is patterned according to any known patterning technique (FIG. 1(f)); a barrier metal film is then formed according to, for instance, the PVD, CVD or ALD film-forming technique (FIG. 1 (g)); a Cu-seed film is formed according to, for instance, the PVD or CVD film-forming technique (FIG. 1(h)); and finally an upper Cu-electrical interconnection film is formed according to the plating method.
The foregoing process as shown in FIG. 1(d-2) fundamentally comprises a selective growth of a film and accordingly, the applicability of this CVD process is dependent upon whether the selectivity of the growth of a film can be ensured or not. In general, a pre-treatment (FIG. 1(d-1)) is carried out to reduce the oxidized film of Cu and to thus obtain clean Cu metal and then a metal film for forming a capping film is formed in order to selectively grow a metal for capping film prior to the formation of the upper Cu-electrical interconnection. As such a pre-treatment, there has conventionally been used, for instance, a treatment such as an H2-annealing treatment, an H2-plasma treatment or a treatment with H-radicals. When these pre-treating methods are used, however, the surface of the electrical insulating film is likewise terminated by H atoms and accordingly, the capping metal grows not only on the Cu-electrical interconnection, but also on the electrical insulating film. For this reason, when the conventional selective CVD process, which comprises such a pre-treating step, is used for the formation of a capping film, a practical problem arises.
When a capping film is formed using, for instance, WF6 as a raw gas according to the selective CVD method after the practice of an H2-annealing treatment or an H2-plasma treatment as the foregoing pre-treatment, a W-film is formed, in the form of a blanket, not only on the Cu-electrical interconnection, but also on the electrical insulating film as will be seen from FIG. 2 or the selectivity of the W-film formation is greatly broken. This is because the surface of the electrical insulating film is terminated by H atoms, activated sites are thus formed on the electrical insulating film surface, WF6 may act on the H atoms to thus form HF, the electrical insulating film is thus etched with the resulting HF and this accordingly impairs the selectivity of the W-film formation. The breakage of the selectivity herein used means such a phenomenon that the material for forming a capping film is deposited on the surface of the electrical insulating film. If the selectivity is broken, a problem arises such that it is necessary to carry out an etch back step and this accordingly leads to the reduction of the advantages of the selective CVD method.